How big is Big enough?

[JoeCrow]

I have a 2002 Mercury 125 hp 2 cycle carburated motor
Anyone know what the largest diameter prop I can use would be?
I'd like to increase the diameter from the present 13" to something larger maybe in the 14" to 15" diameter range
Looks like there's room for a bit more
The 17 to 19 pitch seems to work well for hole shot, but I'd like to gain some top end, so I'm thinking that adding some diameter would help and still maintain the hole shot I have now
Thanks

 

[steelespike]

Re: How big is Big enough?

You don't select a prop specifically by diameter.Diameter is a product of prop design.If you want to improve performance use prop technology,prop design and material.Arbitrarily increasing diameter will most likely actually reduce your speed.Pitch and prop design has the most effect on performance.You can also improve speed with no cost by fine tuning your setup specifically the motor height.Raise the motor a hole at a time to a point just before venting becomes a problem.Be sure it pumps water.Under ideal conditions you could gain 2 or more mph.

 

[BMOLCHANY]

Re: How big is Big enough?

OK with that being said what pitch wouldbe better for heavy loads?

 

[Silvertip]

Re: How big is Big enough?

you need to give us something to work with. Engine manufacturers specify a recommended wide open throttle RPM range. Prop style, material, number of blades, rake, etc., are all design features that one must get right to ensure the engine does indeed run at or near the top end of that rpm band. Diameter is buiilt into the design for a specific series of engines. So there is no one-size fits all when it comes to prop selection. To determine where you need to go you need to be a little more specific about your needs. On one hand you want more top end but don't want to lose hole shot. Think about your car? In high gear is it possible you can have high top speed and still have a dragster? That's what your prop needs to do. Less pitch provides hole shot. more pitch provides speed (but only if the engine can actually twist that much pitch). Happiness is somewhere in between. You need to make some wide open throttle runs with an average load, trimmed out, and then note the maximum observed RPM and speed and the pitch of the prop being used. Then post those numbers. Only then can we provide some insight.